Exhaust systems for light aircraft have for the most part been designed without regard to tuning. Tuning is a characteristic of exhaust systems where the lengths and volumes of the different tubes are designed to resonate in harmony with the exhaust pulses so that a vacuum is formed at the exhaust port at critical times during the exhaust cycle. This is achieved by feeding all the headers to a collector, which allows the exhaust pulses to flow in an even manner, one at a time. Each one helps the one that follows it by leaving a vacuum in its wake. The length of each header must also be sufficient to prevent the sonic pulse from the next cylinder from creating a positive pressure wave before the exhaust valve of the subject cylinder closes.
Exhaust systems have been designed to improve engine performance for four-cylinder engines in cars by insulating exhaust pipes as discussed in U.S. Pat. No. 4,022,019; by tuning the exhaust manifold to match different engine speeds in U.S. Pat. No. 4,731,995; by installing partitioning walls into separate channels of a tailpipe to change the length of each exhaust channel in U.S. Pat. No. 4,815,274; by tuning exhaust systems for two-cycle engines with a Y-shaped hollow exhaust pipe in U.S. Pat. No. 4,835,965; by varying exhaust pipe configurations with valves and shafts coupled together for simultaneous rotation in U.S. Pat. No. 4,869,063 and in U.S. Pat. No. 5,072,583 a third exhaust passage is used to bypass two intermediate exhaust passages to lower exhaust loss at high engine speeds. Thus, it is known to make physical changes to exhaust systems in the form of insulation, partitions, uniquely shaped pipes, extra pipes, valves and couplings which increase engine production costs.
Chemical means for improving engine performance are discussed in U.S. Pat. No. 4,926,634 wherein a homogeneous exhaust gas mixture is produced using lambda controls and two catalysts; in U.S. Pat. No. 5,050,378 air or other gases from the exhaust are admitted to engine cylinders to effectively equalize header lengths and give an additional charge to each cylinder; and in U.S. Pat. No. 5,471,835 an exhaust control system is disclosed wherein tubes and one-way valves are arranged to permit air to be drawn into each pipe for more complete combustion of exhaust gases, thereby reducing emission and major pollutants. None of the references cited above specifically address aircraft engines, and only one reference, U.S. Pat. No. 4,815,274 supra, discloses an exhaust system, including a tailpipe insert designed for a confirmed space; such an insert would be a cost disadvantage.
The probable reason why aircraft exhaust systems have not been tuned, apart from lack of knowledge of the design techniques, is that the space available in the engine cowling is extremely limited, and the tubing lengths required are longer than could conveniently be fitted into the space available.
To compound the problem, the exhaust system has various other functions to perform, such as to attenuate the noise, to provide heat for the cabin, and to provide heated intake air for those engines using carburetors.
The first objective of the present invention is to configure predetermined lengths of exhaust channels to tune an engine.
The second objective of the present invention is to design a tuned exhaust system to fit within a limited space of an engine cowling.
The third objective of the present invention is to design a tuned exhaust system that provides ample heat to an aircraft cabin.
The fourth objective of the present invention is to design a tuned exhaust system that provides heated intake air for an engine carburetor.
The fifth objective of the present invention is to achieve engine performance gains which result in a shortened takeoff distance for aircraft, increased rate of climb to achieve cruising altitudes, added fuel efficiency, while maintaining cooler cylinder head temperatures.
The novelty of this invention relates to an arrangement of the exhaust tubes which allow them both to be tuned to the correct length and to fit in the limited space available, at the same time being able to provide ample cabin heat, carburetor heat and sound attenuation.
History of the invention: The concept was first tried on Apr. 14, 1998 at Daytona Beach International Airport, Florida on a Cherokee 140 aircraft. Refinements were made to the design through November 1998. A prototype exhaust system was displayed at Oshkosh AirVenture on Aug. 1, 1998. A Supplementary Type Certificate was applied for and granted by the Federal Aviation Authority (FAA) on Jan. 7, 1999. First product sold to the public on Mar. 13, 1999.
Further objects and advantages of this invention will be apparent from the following detailed description of a presently preferred embodiment, which is illustrated schematically in the accompanying drawings.